Iranian Journal of Pharmaceutical Research (2016), 15 (Special issue): 29-35 Received: September 2013 Accepted: December 2013
Copyright © 2016 by School of Pharmacy Shaheed Beheshti University of Medical Sciences and Health Services
Original Article
Synthesis and Antibacterial Activity of Novel Hydroxy Semicarbazone Derivatives Elham Hariria, Arash Mahboubib, Mohammad Fathic, Parisa Rahmanib, Kamaleddin Haj Mohammad Ebrahim Tehrania, Mohammad Babaeianb, Vida Mashayekhia and Farzad Kobarfarda* Department of Medicinal Chemistry, School of Pharmacy, Phytochemistry Research Center, Shahid Beheshti University of Medical Sciences, Iran. bDepartment of Pharmaceutics, School of Pharmacy, Shahid Beheshti University of Medical Sciences, Iran. cDepartment of Anestesiology, Shahid Modarres Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran. a
Abstract A series of hydroxyl semicarbazone derivatives of substituted diaryl ketones and acetophenones were synthesized and their structures were confirmed by analytical and spectroscopic methods including elemental analysis, infrared and nuclear magnetic resonance spectroscopy. The derivatives were prepared by a condensation reaction between N-hydroxy semicarbazide and substituted diaryl ketones or acetophenones leading to the desired hydroxysemicarbazones with excellent purity. The synthesized hydrazones were then evaluated for their inhibitory activity against bacterial strains including S. aureus, E. Coli, P. aeruginosa, K. pneumonia and M. luteus. Among the tested derivatives, compounds 2, 6 and 7 exhibited the highest bioactivity. Analysis of the activity data suggests that hydrophilicity is an important factor for the bioactivity of compounds 2 and 6 and also their selectivity over the gram-negative bacteria. Keywords: N-hydroxy semicarbazone; Antibacterial; Broth microdilution assay; Ketones; Hydrophilicity.
Introduction In the drug discovery efforts to combat against microbial infections, (thio) semicarbazones have attracted considerable attentions and many derivatives of this class have been reported to possess promising bioactivity. In addition some (thio) semicarbazones have been marketed in some periods as antimicrobial drugs and some more are under investigations as potential ones. Because of their interference with vital biochemical processes in the living cells such as deoxyribonucleotide synthesis (1), cell wall * Corresponding author: E-mail: [email protected]
biosynthesis (2) and maintaining thiol contents (3), (thio)semicarbazones have been reported to exert antibacterial (4), antimycobacterial (5), anticancer (1), antifungal (6) and antimalarial (7) activities. The general structure of the active (thio) semicarbazones is disclosed in Figure 1, which consists of an aromatic system linked to the (thio) semicarbazone moiety. Working on this structural backbone, many research projects have been conducted to discover novel (thio) semicarbazone derivatives with optimized potency and safety profiles. In a research work carried out by Sriram and his colleagues, some N-hydroxythiosemicarbazones of different aromatic systems were synthesized and tested for their antimycobacterial
of aromatic different system aromatic systems were synthesized and te ve (thio) semicarbazones is disclosed in hydroxythiosemicarbazones Figure 1, which consists of an
antimycobacterial activitybackbone, (8). Among the employed aromatic carbonyl compo ed to the (thio) semicarbazone moiety.their Working on this structural many
condensenovel with (thio) N-hydroxythiosemicarbazide, the order of activity of the tested com arch projects have been conducted to discover semicarbazone derivatives
et al. / IJPR (2016), 15 (Special 29-35 to be Schiff bases of diaryl ketones > acetophenones > aromatic aldehydes wasissue): found h optimized potency Hariri and Esafety profiles.
2).
Figure 1. General structure of active (thio) semicarbazones. X
Figure 2. Antimycobacterial N-hydroxy semicarbazone
ure 1. General structure ofR active X = O, S; Rderivatives = various aromatic = O, S; = various (thio) aromatic semicarbazones. and aliphatic2. substituents. reported by Sriram et al. (8). derivatives reported by Sriram Figure Antimycobacterial N-hydroxy semicarbazone
aliphatic substituents.
(8).
the samples some were dissolved in DMSO-d6 and activity (8). Among the Sriram employed and aromatic n a research work carried out by his NConsidering abovecolleagues, findings and in continuation of our tetramethylsilane (0.05% v/v) as interest internalto study the bio carbonyl compounds to condense thewith roxythiosemicarbazones of different aromatic systems were synthesized for standard. All the were analyzed for N-hydroxythiosemicarbazide, the order of of hydrazone derivatives (9-12),and in tested thecompounds present work we describe the synthe activity of the tested compounds was found to be C, H and N on a Costech model 4010 and agreed r antimycobacterial activity (8). Among the employed aromatic carbonyl compounds to Schiff bases of diaryl ketones > acetophenones > with the proposed structures within ±0.4% of aromatic aldehydes (Figure the tested theoretical values. Hydroxysemicarbazide dense with N-hydroxythiosemicarbazide, the2). order of activity of the compounds (13) and 4,4’-dihydroxybenzophenone Considering the above findings and in found to be Schiff bases of diaryl ketones > acetophenones > aromatic aldehydes (Figure 2,4-dinitrophenylhydrazone (compound 7) (13) continuation of our interest to study the were prepared according to previously reported bioactivity of hydrazone derivatives (9methodologies. 12), in the present work we describe the synthesis and antibacterial activity of some Benzophenone hydroxysemicarbazone (1) N-hydroxysemicarbazones of diaryl ketones Method A. (conventional synthesis) and substituted acetophenones. These Benzophenone (3.64 g, 20 mmol) was derivatives could be considered as bioisoesters added to 50 mL of absolute ethanol and the of active N-hydroxythiosemicarbazones in mixture was heated at 70 °C. Then, a solution which the thiocarbonyl is substituted by of hydroxysemicarbazide (1.82 g, 20 mmol) in carbonyl functional group. The new derivatives ure 2. Antimycobacterial N-hydroxy semicarbazone derivatives reported by Sriram et al. 20 mL of water and 7 drops of glacial acetic could also be viewed as active antimicrobial acid was added dropwise to the solution via a semicarbazones bearing a hydroxyl substitutent dropping funnel. The mixture was heated under at their thioamide nitrogen (Figure 3). Considering the above findings and in continuation of our interest to study the bioactivity reflux for 48 h and then concentrated by vacuum The resulting hydrazone derivatives (9-12), in Experimental the present work we describedistillation. the synthesis and precipitate was filtered and recrystallized from methanol to afford the title compound as white powder (1.27 g, 24%). General Melting points measured using antibacterial activity were of some N-hydroxysemicarbazones of diaryl ketones and substituted Method B. (microwave-assisted synthesis) an Electrothermal 9100 apparatus and are acetophenones. These spectra derivatives could be considered as beaker, bioisoesters of active In a 100 mL benzophenone (3.64 Ng, uncorrected. The Infrared were obtained 20 mmol) dissolved 50 mL offunctional absolute with a Perkin-Elmer 843 spectrometer. hydroxythiosemicarbazones in which Proton the thiocarbonyl is was substitutes byincarbonyl nuclear magnetic resonance (1H NMR) and ethanol by a gentle heating. A solution of 13 viewed as active antimicrobial semicarbazones group. new derivatives could also (be hydroxysemicarbazide (1.82 g, 20 mmol) in carbon The nuclear magnetic resonance C mL of (Figure water and NMR) spectra were determined by a Bruker bearing a hydroxyl substitutent at their thioamide20 nitrogen 3). 7 drops of glacial acetic acid was then added in three portions to the Avance DRX 500 MHz spectrometer and
Figure 3. Design of the novel N-hydroxy semicarbazone derivatives based on the structural elements of active N-hydroxy thiosemicarbazones (left) and semicarbazones (right).
Figure 3. Design of the novel N-hydroxy semicarbazone derivatives based on the structural 30
elements of active N-hydroxy thiosemicarbazones (left) and semicarbazones (right). Experimental General
Antibacterial N-hydroxy semicarbazones
1634 (C=O); 1H NMR (CDCl3/500 MHz): δ 12.40 (s, 1H), 11.95 (s, 1H), 9.9 (s, 1H), 9.27 (s), 7.39 (m, 3H, Ar H), 7.13 (d, J=7.26, 2H, Ar H), 6.51 (d, J=8.78, 1H, Ar H), 6.13 (s, 1H, Ar H), 6.03 (dd, J=8.79, J=2.33, Ar H); Anal. Calcd for C14H13N3O4 (287.27): C, 58.53; H, 4.56; N, 14.63. Found: C, 58.40; H, 4.57; N, 14.65.
benzophenone solution. After addition of each portion, the beaker was placed in a microwave reactor set at 600 w for five 30-second periods. After the completion of the procedure, the beaker was cooled at room temperature and the resulting precipitate was filtered. The crude was recrystallized from 2-propanol to afford the title compound. White powder (3.31 g, 65%): mp 162.5165°C; IR (KBr): 3200 (OH), 3350 (NH), 1668 (C=O); 1H NMR (CDCl3/500 MHz): δ 8.30 (br s, 1H), 7.85 (br s, 1H), 7.65-7.36 (m, 5H, Ar H), 7.21-7.17 (m, 3H, Ar H), 7.12 (d, J=8.15, Ar H); Anal. Calcd for C14H13N3O2 (255.27): C, 65.87; H, 5.13; N, 16.46. Found: C, 65.71; H, 5.14; N, 16.50. (2)
4,4’-Dihydroxybenzophenone hydroxyl semicarbazone (4) The reaction was carried out as described for compound 1 (method A), except that an appropriate amount of molecular sieve was added to the reaction mixture. After heating the mixture under reflux for 24 h, stirring for further 48 h and typical workup as described earlier, the title compound was obtained. Yellow powder (3.50 g, 60%): mp 141144 °C; IR (KBr): 3200 (OH), 1630 (C=O); 1H NMR (CDCl3/500 MHz): δ 12.20 (s, 1H), 10.72 (s, 1H), 7.61 (m, 3H, Ar H), 7.54 (m, 2H, Ar H), 7.37 (d, J=8.66, 1H, Ar H), 6.38 (m, 2H, Ar H); Anal. Calcd for C14H13N3O4 (287.27): C, 58.53; H, 4.56; N, 14.63. Found: C, 58.60; H, 4.56; N, 14.69.
Benzoylbenzoic acid hydroxyl semicarbazone
After addition of the regents as described for compound 1 (method A), the mixture was heated under reflux for 1 h and then stirred at room temperature for 24 h. The mixture was concentrated by vacuum distillation and refrigerated for 1 h. The precipitate was filtered and recrystallized form methanol. White powder (4.24 g, 72%): mp 230231 °C; IR (KBr): 3000 (OH), 3160 (NH), 1670 (C=O); 1H NMR (DMSO-d6/500 MHz): δ 7.56-7.65 (m, 6H, Ar H), 7.68 (m, 1H, Ar H), 7.92 (m, 2H, Ar H), 8.36 (1H, s, Ar H), 12.88 (s, 1H); 13C NMR (CDCl3/125 MHz): δ 160.8, 148.3, 135.4, 133.6, 131.8, 130.0, 129.5, 128.7, 127.1, 77.8, 77.5, 77.3, 40.5, 40.4, 40.2; Anal. Calcd for C15H13N3O4 (299.28): C, 60.20; H, 4.38; N, 14.04. Found: C, 60.29; H, 4.38; N, 14.00.
Acetophenone hydroxysemicarbazone (5) After addition of the regents as described for compound 1 (method A), the mixture was heated under reflux for 1 h and then stirred at room temperature for 1 h. The precipitate thus formed was filtered and recrystallized form 2-butanol. Yellow powder (2.90 g, 75%): mp 144-146 °C; IR (KBr): 3300 (NH), 3200 (OH), 1680 (C=O); 1H NMR (CDCl3/500 MHz): δ 8.61 (br s, 1H), 8.35 (br s, 1H), 7.63 (d, J=6.96, 2H, Ar H), 7.35 (m, 1H, Ar H), 7.29 (m, 2H, Ar H), 2.15 (s, 3H, CH3), 2.1 (s); Anal. Calcd for C9H11N3O2 (193.20): C, 55.95; H, 5.74; N, 21.75. Found: C, 55.87; H, 5.75; N, 21.71.
2,4-Dihydroxybenzophenone hydroxyl semicarbazone (3) After addition of the regents as described for compound 1 (method A), the mixture was heated under reflux for 24 h and then stirred at room temperature for 48 h. The mixture was concentrated by vacuum distillation and refrigerated for 1 h. The precipitate was filtered and the title compound with acceptable purity was obtained. Yellow powder (4.24 g, 74%): mp 184186 °C; IR (KBr): 3100 (OH), 3200 (NH),
(6)
Methoxyacetophenone hydroxyl semicarbazone
After addition of the regents as described for compound 1 (method A), the mixture was heated under reflux for 1 h and then stirred at room temperature for 2 h. The precipitate thus formed was filtered and the title compound with acceptable purity was obtained. Light brown powder (3.25 g, 73%): mp 18131
Hariri E et al. / IJPR (2016), 15 (Special issue): 29-35
Scheme 1. Synthesis of N-hydroxy 1-6. Scheme 1. semicarbazones Synthesis of N-hydroxy semicarbazones 1-6. 182.5 °C; IR (KBr): 3360, 3220, 2840, 1660; 1H structure andMixture purity ofofE/Z theisomers synthesized derivatives were evaluated using different NMR The (CDCl /500 MHz): 3 δ 8.15 (s, 1H),methods 7.94 (d, J=8.55, 2H,IR, Ar NMR H), 7.65and elemental analysis. In the IR spectra, stretch analytical such as Where ODa, ODb and ODc are the (d, J=8.70, 2H, Ar H), 6.96 (d, J=8.86, 2H, Ar vibrations relating to O-H and N-H bonds in the hydroxysemicarbazone moiety was observed optical density of the solutions containing H), 6.90 (d, J=8.95, 2H, Ar H), 3.87 (s, 3H, microorganisms and test compounds, only OCH3), 3.85 (s, 3H, OCH3), 2.19 (s, 3H, CH3), as broad weak bands in 3100-3350 region of spectra. In the carbonyl region of the spectra, the 2.09 (s, 3H, CH3); Anal. Calcd for C10H13N3O3 test compounds and only microorganisms (223.23): C, 53.80; H, 5.87; N, 18.82. Found: C, respectively. was defined as the C=O stretch band appeared in 1630-1680 with mediumICto50 strong intensity. Thelowest relative concentration of the test compound in which 53.91; H, 5.86; N, 118.79. complexity of H NMR spectra of the synthesized compounds reflects the fact that due to the the bacterial growth was completely inhibited. and carbon vancomycin were useddifferent as In-vitro evaluation of antibacterial activity rigidity of C=N bond, the two substituents onAmikacin the ketimine give rise to two standard antibiotics. It is notable that each assay Compounds were assessed for their sets of hydrogens. Thisbroth is observable in the 1was H NMR spectra of compounds 1 and 4, where performed as duplicates. antibacterial activity by microdilution method. The strains used were S. aureus ATCC the aryl substituents on the ketimine carbon are the same (two phenyl rings in compound 1 6538, E. Coli ATCC 8439, P. aeruginosa ATCC Results and Discussion 2097, K. pneumonia ATCC 10031 and M. luteus and two p-hydroxyphenyl rings in compound 4). In contrast to benzophenone and 4,4’Chemistry ATCC 9341. All the strains were cultured in dihydroxybenzophenone in which the and hydrogensAs on one ring areinchemical disclosed scheme shift 1, equivalent the final to Soybean Casein Digest Agar (SCDA) derivatives prepared a Schiff base after overnight incubation were diluted to other 0.5 ring, NMR spectra by of compounds 1 and their corresponding hydrogens on the in the 1Hwere formation reaction. The condensation between McFarland turbidity standards. 4Different the chemical shifts for theofhydrogens of each ringsemicarbazide is different from those on ketones the other concentrations the test (thio) and aromatic andring. compounds (10 two µL sets of each) were added to arealdehydes of an acidicofcatalyst 1 As a result, of double doublets observedininthe thepresence aromatic region H NMR often leads to stable crystalline products in good 96 well plates. Then, 80 µL of Muller Hinton to excellent yields. However, cases that (thio) for Broth (MHB) medium and of microbial spectra of compound 4. 10 E/ZµLisomerism as another outcome of C=N bond in rigidity accounts semicarbazones of diaryl ketones are desired, a suspensions were added to each well. The final the complexity the 1H NMR spectrain of compounds 2, 3, reaction 5 and 6could where two different more challenging be anticipated concentration of theofmicrobial suspensions 7 andFor mayinstance, need some modifications to force the each well was are 1.5 present × 10 cfu/mL. The platescarbon. substituents on the ketimine compound 3 could be assumed were sealed to minimize the evaporation of the reaction to proceed. This happens because the two as twoand different isomers (Figure The aryl substituents in each isomer “feel” different medium incubated at 35 °C for 24 4). h. The π-donating aromatic systems make their adjacent optical density of the fields wells and was ifread 580 carbonyl group lesssample, reactive atowards different 1 external magnetic bothat isomers are present in the complex H NMR nucleophiles. In this project, the acetophenone nm using an ELISA reader spectrophotometer 1 spectra willThe notinhibitory be surprising. Interestingly, H NMR5spectra of compound indicates derivatives and 6 were prepared in 6good yields the (TECAN-SP). concentration (IC) the and the reaction time was as short as 2-3 h. in each well was calculated by the following presence of E/Z isomers with 1:1 proportion. However, Our findings are supported by some reports in formula: for preparing compounds 1-4, for the
the literature on the E/Z isomerism of hydrazone derivatives of diaryl ketones. For instance 32
Rihardson et al. in their published work have obtained some benzoylpyridine thiosemicarbazone derivatives as a mixture of E/Z isomers (14).
Antibacterial N-hydroxy semicarbazones
Figure 4. Structures of E (right) and Z (left) isomers of compound 3.
Figure 4. Structures of E (right) and Z (left) isomers of compound 3.
spectra of compounds 2, 3, 5 and 6 where two reasons discussed above, some modifications were mandatory. For instance, synthesis of different substituents are present on the ketimine compound 1 in a microwave reactor proved carbon. For instance, compound 3 could be Biological activity to be more efficient than conventional method assumed as two different isomers (Figure 4). The (see experimental). The synthesis yield for the aryl substituents in each isomeractivity “feel” different The synthesized derivatives were evaluated for their antibacterial against 6 other benzophenone derivatives was improved external magnetic fields and if both isomers are strains andtimethe are listed in table 1. 1HInNMR addition bydifferent increasing the reaction and MIC additionvalues of present in the sample, a complex spectra to 1 molecular sieve to the reaction mixture. 1-6, we will not interested be surprising. H NMR of hydroxysemicarbazone derivatives were in Interestingly, bioactivity the evaluation The structure and purity of the synthesized spectra of compound 6 indicates the presence of compoundwere 7. This compound been reported to exhibit activity against some derivatives evaluated usinghasdifferent E/Z isomers withantitumor 1:1 proportion. Our findings analytical methods such Since as IR,many NMRantitumor and are supportedhave by some in the literature cancer cell lines (15). derivatives also reports been reported to possess elemental analysis. In the IR spectra, stretch on the E/Z isomerism of hydrazone derivatives antimicrobial antimicrobial assessment of compound as a vibrations relatingactivity to O-H(16-19), and N-H bonds of activity diaryl ketones. For instance Rihardson et7 al. in the hydroxysemicarbazone moiety was in their published work have obtained some previously established anticancer agent in order to evaluate itsthiosemicarbazone potential to be considered observed as broad weak bands in 3100-3350 benzoylpyridine derivativesas a region In the carbonyl of asseems a mixturelogical. of E/Z isomers (14). lead of inspectra. antimicrobial drug region discovery Furthermore, since the the spectra, the C=O stretch band appeared in hydroxysemicarbazone replaced by a Biological phenylhydrazine 1630-1680 with medium tomoiety strongis intensity. activity derivative in compound 7, 1 The complexity H to NMR spectra synthesized derivatives were in evaluated thisrelative derivative can be of used assess the role of The hydroxysemicarbazone moiety the tested of the synthesized compounds reflects the for their antibacterial activity against 6 different factderivatives. that due to the rigidity of C=N bond, the strains and the MIC values are listed in table 1. two substituents on the ketimine carbon give In addition to hydroxysemicarbazone derivatives As it appears in table 1, compound 7 showed the highest activity against P. aeruginosa rise to two different sets of hydrogens. This is 1-6, we were interested in bioactivity evaluation 1 observable in thewith H NMR compound 7. the Thissubstituted compoundbenzophenone has been and E. Coli IC50 spectra values of ofcompounds 62.5 and 31.25ofµg/mL. Among 1 and 4, where the aryl substituents on the reported to exhibit antitumor activity against derivatives, 2 with 2-carboxy was the active derivative especially ketimine carboncompound are the same (two phenyl ringssubstituent some cancer cell most lines (15). Since many antitumor in against compound 1 and two p-hydroxyphenyl have and alsoK. been reported to possess gram-negative strains such asrings E. Coli,derivatives P. aeruginosa pneumonia. While the 4in compound 4). In contrast to benzophenone antimicrobial activity (16-19), antimicrobial derivativein6 showed against Coli, P. aeruginosa, andmethoxyacetophenone 4,4’-dihydroxybenzophenone which moderate activity activity assessment of E. compound 7 as a the hydrogens on one ring are chemical shift previously established anticancer agent in order the acetophenone derivative 5 did not show any activity at 1000 µg/mL concentration. equivalent to their corresponding hydrogens to evaluate its potential to be considered as a lead on theBased other on ring, the 1H of NMR antimicrobial drugthe discovery seems logical. theinanalysis the spectra activityofdata, in it is suggested that presence of hydrophilic compounds 1 and 4 the chemical shifts for the Furthermore, since the hydroxysemicarbazone substituents on the ring is important for antibacterial activity. evident from the hydrogens of each ring aromatic is different from those moiety is replaced by a Asphenylhydrazine onactivity the otherdata, ring. As a result, two sets of double derivative in compound 7, thishigher derivative can bethan 2-carboxy substituted benzophenone derivative 2 showed activity doublets are observed in the aromatic region of used to assess the role of hydroxysemicarbazone 1 the unsubstituted benzophenone derivative 1. Similarly, 4-methoxysubstituted acetophenone H NMR spectra of compound 4. E/Z isomerism moiety in the tested derivatives. as another outcome of C=N bond rigidity As it appears in table 1, compound 7 showed accounts for the complexity of the 1H NMR the highest activity against P. aeruginosa and 33
Hariri E et al. / IJPR (2016), 15 (Special issue): 29-35
Table 1. Antibacterial activity of the synthesized derivatives. MIC (µg/mL)
Compounds 1
> 1000
500
250
250
1000
2
500
125
125
62.5
500
3
125
> 1000
500
> 1000
250
4
125
1000
500
1000
62.5
5
1000
1000
1000
1000
1000
6
500
500
250
125
500
7
500
125
62.5
31.25
500
4
0.5
2
1
2
≤4
≤4
32
≤4
≤2
+
+
+
+
+
Amikacin Vancomycin DMSO
Conclusion
E. Coli with IC50 values of 62.5 and 31.25 µg/ mL. Among the substituted benzophenone derivatives, compound 2 with 2-carboxy substituent was the most active derivative especially against gram-negative strains such as E. Coli, P. aeruginosa and K. pneumonia. While the 4-methoxyacetophenone derivative 6 showed moderate activity against E. Coli, P. aeruginosa, the acetophenone derivative 5 did not show any activity at 1000 µg/mL concentration. Based on the analysis of the activity data, it is suggested that the presence of hydrophilic substituents on the aromatic ring is important for antibacterial activity. As evident from the activity data, 2-carboxy substituted benzophenone derivative 2 showed higher activity than the unsubstituted benzophenone derivative 1. Similarly, 4-methoxysubstituted acetophenone derivative 6 exhibited higher potency than its unsubstituted analog (compound 5). It is notable that compounds 2 and 5 demonstrated some selectivity through inhibition of gram-negative bacteria. This might be due to higher penetration of these hydrophilic derivatives inside the gramnegative bacteria through their hydrophilic porin channels which leads to availability of higher toxic concentrations of the tested derivatives inside the microorganism. Besides the hydroxysemicarbazones 1-6, phenylhydrazone derivatives 7 exhibited promising antibacterial activity which gives us a new opportunity to conduct more focused studies on the antibacterial activity of other phenylhydrazone derivatives of diaryl systems.
In this study, based on the pharmacophoric elements of active hydroxythiosemicarbazones and semicarbazones, a group of hydroxysemicarbazone derivatives were synthesized. The synthetic yields of the diaryl ketone derivatives improved by employing either microwave technique, increasing the reaction time or addition of molecular sieve. The derivatives were analyzed by different spectroscopic methods and among them, the 1H NMR data confirmed the formation of some derivatives as a mixture of E/Z isomers. The antibacterial activity of the synthesized derivatives was assessed against 6 pathogenic bacterial strains and compounds 2, 6 and 7 exhibited the highest activity. Notably, the activity of these derivatives was more pronounced against gram-negative bacteria. The facts that both compounds 2 and 7 possess hydrophilic substituents and selectivity over gram-negative bacteria, suggest their possible higher capability to pass through the porin channels available on the cell wall of the microorganism as a reasonable explanation for their antibacterial activity. References (1) Kalinowski DS and Richardson DR. Future of toxicologys–Iron chelators and differing modes of action and toxicity: The changing face of iron chelation therapy. Chem. Res. Toxicol. (2007) 20: 715-720. (2) Dover LG, Alahari A, Gratraud P, Gomes JM and Bhowruth VE. EthA, a common activator of
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Copyright © 2016 by School of Pharmacy Shaheed Beheshti University of Medical Sciences and Health Services
Original Article
Synthesis and Antibacterial Activity of Novel Hydroxy Semicarbazone Derivatives Elham Hariria, Arash Mahboubib, Mohammad Fathic, Parisa Rahmanib, Kamaleddin Haj Mohammad Ebrahim Tehrania, Mohammad Babaeianb, Vida Mashayekhia and Farzad Kobarfarda* Department of Medicinal Chemistry, School of Pharmacy, Phytochemistry Research Center, Shahid Beheshti University of Medical Sciences, Iran. bDepartment of Pharmaceutics, School of Pharmacy, Shahid Beheshti University of Medical Sciences, Iran. cDepartment of Anestesiology, Shahid Modarres Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran. a
Abstract A series of hydroxyl semicarbazone derivatives of substituted diaryl ketones and acetophenones were synthesized and their structures were confirmed by analytical and spectroscopic methods including elemental analysis, infrared and nuclear magnetic resonance spectroscopy. The derivatives were prepared by a condensation reaction between N-hydroxy semicarbazide and substituted diaryl ketones or acetophenones leading to the desired hydroxysemicarbazones with excellent purity. The synthesized hydrazones were then evaluated for their inhibitory activity against bacterial strains including S. aureus, E. Coli, P. aeruginosa, K. pneumonia and M. luteus. Among the tested derivatives, compounds 2, 6 and 7 exhibited the highest bioactivity. Analysis of the activity data suggests that hydrophilicity is an important factor for the bioactivity of compounds 2 and 6 and also their selectivity over the gram-negative bacteria. Keywords: N-hydroxy semicarbazone; Antibacterial; Broth microdilution assay; Ketones; Hydrophilicity.
Introduction In the drug discovery efforts to combat against microbial infections, (thio) semicarbazones have attracted considerable attentions and many derivatives of this class have been reported to possess promising bioactivity. In addition some (thio) semicarbazones have been marketed in some periods as antimicrobial drugs and some more are under investigations as potential ones. Because of their interference with vital biochemical processes in the living cells such as deoxyribonucleotide synthesis (1), cell wall * Corresponding author: E-mail: [email protected]
biosynthesis (2) and maintaining thiol contents (3), (thio)semicarbazones have been reported to exert antibacterial (4), antimycobacterial (5), anticancer (1), antifungal (6) and antimalarial (7) activities. The general structure of the active (thio) semicarbazones is disclosed in Figure 1, which consists of an aromatic system linked to the (thio) semicarbazone moiety. Working on this structural backbone, many research projects have been conducted to discover novel (thio) semicarbazone derivatives with optimized potency and safety profiles. In a research work carried out by Sriram and his colleagues, some N-hydroxythiosemicarbazones of different aromatic systems were synthesized and tested for their antimycobacterial
of aromatic different system aromatic systems were synthesized and te ve (thio) semicarbazones is disclosed in hydroxythiosemicarbazones Figure 1, which consists of an
antimycobacterial activitybackbone, (8). Among the employed aromatic carbonyl compo ed to the (thio) semicarbazone moiety.their Working on this structural many
condensenovel with (thio) N-hydroxythiosemicarbazide, the order of activity of the tested com arch projects have been conducted to discover semicarbazone derivatives
et al. / IJPR (2016), 15 (Special 29-35 to be Schiff bases of diaryl ketones > acetophenones > aromatic aldehydes wasissue): found h optimized potency Hariri and Esafety profiles.
2).
Figure 1. General structure of active (thio) semicarbazones. X
Figure 2. Antimycobacterial N-hydroxy semicarbazone
ure 1. General structure ofR active X = O, S; Rderivatives = various aromatic = O, S; = various (thio) aromatic semicarbazones. and aliphatic2. substituents. reported by Sriram et al. (8). derivatives reported by Sriram Figure Antimycobacterial N-hydroxy semicarbazone
aliphatic substituents.
(8).
the samples some were dissolved in DMSO-d6 and activity (8). Among the Sriram employed and aromatic n a research work carried out by his NConsidering abovecolleagues, findings and in continuation of our tetramethylsilane (0.05% v/v) as interest internalto study the bio carbonyl compounds to condense thewith roxythiosemicarbazones of different aromatic systems were synthesized for standard. All the were analyzed for N-hydroxythiosemicarbazide, the order of of hydrazone derivatives (9-12),and in tested thecompounds present work we describe the synthe activity of the tested compounds was found to be C, H and N on a Costech model 4010 and agreed r antimycobacterial activity (8). Among the employed aromatic carbonyl compounds to Schiff bases of diaryl ketones > acetophenones > with the proposed structures within ±0.4% of aromatic aldehydes (Figure the tested theoretical values. Hydroxysemicarbazide dense with N-hydroxythiosemicarbazide, the2). order of activity of the compounds (13) and 4,4’-dihydroxybenzophenone Considering the above findings and in found to be Schiff bases of diaryl ketones > acetophenones > aromatic aldehydes (Figure 2,4-dinitrophenylhydrazone (compound 7) (13) continuation of our interest to study the were prepared according to previously reported bioactivity of hydrazone derivatives (9methodologies. 12), in the present work we describe the synthesis and antibacterial activity of some Benzophenone hydroxysemicarbazone (1) N-hydroxysemicarbazones of diaryl ketones Method A. (conventional synthesis) and substituted acetophenones. These Benzophenone (3.64 g, 20 mmol) was derivatives could be considered as bioisoesters added to 50 mL of absolute ethanol and the of active N-hydroxythiosemicarbazones in mixture was heated at 70 °C. Then, a solution which the thiocarbonyl is substituted by of hydroxysemicarbazide (1.82 g, 20 mmol) in carbonyl functional group. The new derivatives ure 2. Antimycobacterial N-hydroxy semicarbazone derivatives reported by Sriram et al. 20 mL of water and 7 drops of glacial acetic could also be viewed as active antimicrobial acid was added dropwise to the solution via a semicarbazones bearing a hydroxyl substitutent dropping funnel. The mixture was heated under at their thioamide nitrogen (Figure 3). Considering the above findings and in continuation of our interest to study the bioactivity reflux for 48 h and then concentrated by vacuum The resulting hydrazone derivatives (9-12), in Experimental the present work we describedistillation. the synthesis and precipitate was filtered and recrystallized from methanol to afford the title compound as white powder (1.27 g, 24%). General Melting points measured using antibacterial activity were of some N-hydroxysemicarbazones of diaryl ketones and substituted Method B. (microwave-assisted synthesis) an Electrothermal 9100 apparatus and are acetophenones. These spectra derivatives could be considered as beaker, bioisoesters of active In a 100 mL benzophenone (3.64 Ng, uncorrected. The Infrared were obtained 20 mmol) dissolved 50 mL offunctional absolute with a Perkin-Elmer 843 spectrometer. hydroxythiosemicarbazones in which Proton the thiocarbonyl is was substitutes byincarbonyl nuclear magnetic resonance (1H NMR) and ethanol by a gentle heating. A solution of 13 viewed as active antimicrobial semicarbazones group. new derivatives could also (be hydroxysemicarbazide (1.82 g, 20 mmol) in carbon The nuclear magnetic resonance C mL of (Figure water and NMR) spectra were determined by a Bruker bearing a hydroxyl substitutent at their thioamide20 nitrogen 3). 7 drops of glacial acetic acid was then added in three portions to the Avance DRX 500 MHz spectrometer and
Figure 3. Design of the novel N-hydroxy semicarbazone derivatives based on the structural elements of active N-hydroxy thiosemicarbazones (left) and semicarbazones (right).
Figure 3. Design of the novel N-hydroxy semicarbazone derivatives based on the structural 30
elements of active N-hydroxy thiosemicarbazones (left) and semicarbazones (right). Experimental General
Antibacterial N-hydroxy semicarbazones
1634 (C=O); 1H NMR (CDCl3/500 MHz): δ 12.40 (s, 1H), 11.95 (s, 1H), 9.9 (s, 1H), 9.27 (s), 7.39 (m, 3H, Ar H), 7.13 (d, J=7.26, 2H, Ar H), 6.51 (d, J=8.78, 1H, Ar H), 6.13 (s, 1H, Ar H), 6.03 (dd, J=8.79, J=2.33, Ar H); Anal. Calcd for C14H13N3O4 (287.27): C, 58.53; H, 4.56; N, 14.63. Found: C, 58.40; H, 4.57; N, 14.65.
benzophenone solution. After addition of each portion, the beaker was placed in a microwave reactor set at 600 w for five 30-second periods. After the completion of the procedure, the beaker was cooled at room temperature and the resulting precipitate was filtered. The crude was recrystallized from 2-propanol to afford the title compound. White powder (3.31 g, 65%): mp 162.5165°C; IR (KBr): 3200 (OH), 3350 (NH), 1668 (C=O); 1H NMR (CDCl3/500 MHz): δ 8.30 (br s, 1H), 7.85 (br s, 1H), 7.65-7.36 (m, 5H, Ar H), 7.21-7.17 (m, 3H, Ar H), 7.12 (d, J=8.15, Ar H); Anal. Calcd for C14H13N3O2 (255.27): C, 65.87; H, 5.13; N, 16.46. Found: C, 65.71; H, 5.14; N, 16.50. (2)
4,4’-Dihydroxybenzophenone hydroxyl semicarbazone (4) The reaction was carried out as described for compound 1 (method A), except that an appropriate amount of molecular sieve was added to the reaction mixture. After heating the mixture under reflux for 24 h, stirring for further 48 h and typical workup as described earlier, the title compound was obtained. Yellow powder (3.50 g, 60%): mp 141144 °C; IR (KBr): 3200 (OH), 1630 (C=O); 1H NMR (CDCl3/500 MHz): δ 12.20 (s, 1H), 10.72 (s, 1H), 7.61 (m, 3H, Ar H), 7.54 (m, 2H, Ar H), 7.37 (d, J=8.66, 1H, Ar H), 6.38 (m, 2H, Ar H); Anal. Calcd for C14H13N3O4 (287.27): C, 58.53; H, 4.56; N, 14.63. Found: C, 58.60; H, 4.56; N, 14.69.
Benzoylbenzoic acid hydroxyl semicarbazone
After addition of the regents as described for compound 1 (method A), the mixture was heated under reflux for 1 h and then stirred at room temperature for 24 h. The mixture was concentrated by vacuum distillation and refrigerated for 1 h. The precipitate was filtered and recrystallized form methanol. White powder (4.24 g, 72%): mp 230231 °C; IR (KBr): 3000 (OH), 3160 (NH), 1670 (C=O); 1H NMR (DMSO-d6/500 MHz): δ 7.56-7.65 (m, 6H, Ar H), 7.68 (m, 1H, Ar H), 7.92 (m, 2H, Ar H), 8.36 (1H, s, Ar H), 12.88 (s, 1H); 13C NMR (CDCl3/125 MHz): δ 160.8, 148.3, 135.4, 133.6, 131.8, 130.0, 129.5, 128.7, 127.1, 77.8, 77.5, 77.3, 40.5, 40.4, 40.2; Anal. Calcd for C15H13N3O4 (299.28): C, 60.20; H, 4.38; N, 14.04. Found: C, 60.29; H, 4.38; N, 14.00.
Acetophenone hydroxysemicarbazone (5) After addition of the regents as described for compound 1 (method A), the mixture was heated under reflux for 1 h and then stirred at room temperature for 1 h. The precipitate thus formed was filtered and recrystallized form 2-butanol. Yellow powder (2.90 g, 75%): mp 144-146 °C; IR (KBr): 3300 (NH), 3200 (OH), 1680 (C=O); 1H NMR (CDCl3/500 MHz): δ 8.61 (br s, 1H), 8.35 (br s, 1H), 7.63 (d, J=6.96, 2H, Ar H), 7.35 (m, 1H, Ar H), 7.29 (m, 2H, Ar H), 2.15 (s, 3H, CH3), 2.1 (s); Anal. Calcd for C9H11N3O2 (193.20): C, 55.95; H, 5.74; N, 21.75. Found: C, 55.87; H, 5.75; N, 21.71.
2,4-Dihydroxybenzophenone hydroxyl semicarbazone (3) After addition of the regents as described for compound 1 (method A), the mixture was heated under reflux for 24 h and then stirred at room temperature for 48 h. The mixture was concentrated by vacuum distillation and refrigerated for 1 h. The precipitate was filtered and the title compound with acceptable purity was obtained. Yellow powder (4.24 g, 74%): mp 184186 °C; IR (KBr): 3100 (OH), 3200 (NH),
(6)
Methoxyacetophenone hydroxyl semicarbazone
After addition of the regents as described for compound 1 (method A), the mixture was heated under reflux for 1 h and then stirred at room temperature for 2 h. The precipitate thus formed was filtered and the title compound with acceptable purity was obtained. Light brown powder (3.25 g, 73%): mp 18131
Hariri E et al. / IJPR (2016), 15 (Special issue): 29-35
Scheme 1. Synthesis of N-hydroxy 1-6. Scheme 1. semicarbazones Synthesis of N-hydroxy semicarbazones 1-6. 182.5 °C; IR (KBr): 3360, 3220, 2840, 1660; 1H structure andMixture purity ofofE/Z theisomers synthesized derivatives were evaluated using different NMR The (CDCl /500 MHz): 3 δ 8.15 (s, 1H),methods 7.94 (d, J=8.55, 2H,IR, Ar NMR H), 7.65and elemental analysis. In the IR spectra, stretch analytical such as Where ODa, ODb and ODc are the (d, J=8.70, 2H, Ar H), 6.96 (d, J=8.86, 2H, Ar vibrations relating to O-H and N-H bonds in the hydroxysemicarbazone moiety was observed optical density of the solutions containing H), 6.90 (d, J=8.95, 2H, Ar H), 3.87 (s, 3H, microorganisms and test compounds, only OCH3), 3.85 (s, 3H, OCH3), 2.19 (s, 3H, CH3), as broad weak bands in 3100-3350 region of spectra. In the carbonyl region of the spectra, the 2.09 (s, 3H, CH3); Anal. Calcd for C10H13N3O3 test compounds and only microorganisms (223.23): C, 53.80; H, 5.87; N, 18.82. Found: C, respectively. was defined as the C=O stretch band appeared in 1630-1680 with mediumICto50 strong intensity. Thelowest relative concentration of the test compound in which 53.91; H, 5.86; N, 118.79. complexity of H NMR spectra of the synthesized compounds reflects the fact that due to the the bacterial growth was completely inhibited. and carbon vancomycin were useddifferent as In-vitro evaluation of antibacterial activity rigidity of C=N bond, the two substituents onAmikacin the ketimine give rise to two standard antibiotics. It is notable that each assay Compounds were assessed for their sets of hydrogens. Thisbroth is observable in the 1was H NMR spectra of compounds 1 and 4, where performed as duplicates. antibacterial activity by microdilution method. The strains used were S. aureus ATCC the aryl substituents on the ketimine carbon are the same (two phenyl rings in compound 1 6538, E. Coli ATCC 8439, P. aeruginosa ATCC Results and Discussion 2097, K. pneumonia ATCC 10031 and M. luteus and two p-hydroxyphenyl rings in compound 4). In contrast to benzophenone and 4,4’Chemistry ATCC 9341. All the strains were cultured in dihydroxybenzophenone in which the and hydrogensAs on one ring areinchemical disclosed scheme shift 1, equivalent the final to Soybean Casein Digest Agar (SCDA) derivatives prepared a Schiff base after overnight incubation were diluted to other 0.5 ring, NMR spectra by of compounds 1 and their corresponding hydrogens on the in the 1Hwere formation reaction. The condensation between McFarland turbidity standards. 4Different the chemical shifts for theofhydrogens of each ringsemicarbazide is different from those on ketones the other concentrations the test (thio) and aromatic andring. compounds (10 two µL sets of each) were added to arealdehydes of an acidicofcatalyst 1 As a result, of double doublets observedininthe thepresence aromatic region H NMR often leads to stable crystalline products in good 96 well plates. Then, 80 µL of Muller Hinton to excellent yields. However, cases that (thio) for Broth (MHB) medium and of microbial spectra of compound 4. 10 E/ZµLisomerism as another outcome of C=N bond in rigidity accounts semicarbazones of diaryl ketones are desired, a suspensions were added to each well. The final the complexity the 1H NMR spectrain of compounds 2, 3, reaction 5 and 6could where two different more challenging be anticipated concentration of theofmicrobial suspensions 7 andFor mayinstance, need some modifications to force the each well was are 1.5 present × 10 cfu/mL. The platescarbon. substituents on the ketimine compound 3 could be assumed were sealed to minimize the evaporation of the reaction to proceed. This happens because the two as twoand different isomers (Figure The aryl substituents in each isomer “feel” different medium incubated at 35 °C for 24 4). h. The π-donating aromatic systems make their adjacent optical density of the fields wells and was ifread 580 carbonyl group lesssample, reactive atowards different 1 external magnetic bothat isomers are present in the complex H NMR nucleophiles. In this project, the acetophenone nm using an ELISA reader spectrophotometer 1 spectra willThe notinhibitory be surprising. Interestingly, H NMR5spectra of compound indicates derivatives and 6 were prepared in 6good yields the (TECAN-SP). concentration (IC) the and the reaction time was as short as 2-3 h. in each well was calculated by the following presence of E/Z isomers with 1:1 proportion. However, Our findings are supported by some reports in formula: for preparing compounds 1-4, for the
the literature on the E/Z isomerism of hydrazone derivatives of diaryl ketones. For instance 32
Rihardson et al. in their published work have obtained some benzoylpyridine thiosemicarbazone derivatives as a mixture of E/Z isomers (14).
Antibacterial N-hydroxy semicarbazones
Figure 4. Structures of E (right) and Z (left) isomers of compound 3.
Figure 4. Structures of E (right) and Z (left) isomers of compound 3.
spectra of compounds 2, 3, 5 and 6 where two reasons discussed above, some modifications were mandatory. For instance, synthesis of different substituents are present on the ketimine compound 1 in a microwave reactor proved carbon. For instance, compound 3 could be Biological activity to be more efficient than conventional method assumed as two different isomers (Figure 4). The (see experimental). The synthesis yield for the aryl substituents in each isomeractivity “feel” different The synthesized derivatives were evaluated for their antibacterial against 6 other benzophenone derivatives was improved external magnetic fields and if both isomers are strains andtimethe are listed in table 1. 1HInNMR addition bydifferent increasing the reaction and MIC additionvalues of present in the sample, a complex spectra to 1 molecular sieve to the reaction mixture. 1-6, we will not interested be surprising. H NMR of hydroxysemicarbazone derivatives were in Interestingly, bioactivity the evaluation The structure and purity of the synthesized spectra of compound 6 indicates the presence of compoundwere 7. This compound been reported to exhibit activity against some derivatives evaluated usinghasdifferent E/Z isomers withantitumor 1:1 proportion. Our findings analytical methods such Since as IR,many NMRantitumor and are supportedhave by some in the literature cancer cell lines (15). derivatives also reports been reported to possess elemental analysis. In the IR spectra, stretch on the E/Z isomerism of hydrazone derivatives antimicrobial antimicrobial assessment of compound as a vibrations relatingactivity to O-H(16-19), and N-H bonds of activity diaryl ketones. For instance Rihardson et7 al. in the hydroxysemicarbazone moiety was in their published work have obtained some previously established anticancer agent in order to evaluate itsthiosemicarbazone potential to be considered observed as broad weak bands in 3100-3350 benzoylpyridine derivativesas a region In the carbonyl of asseems a mixturelogical. of E/Z isomers (14). lead of inspectra. antimicrobial drug region discovery Furthermore, since the the spectra, the C=O stretch band appeared in hydroxysemicarbazone replaced by a Biological phenylhydrazine 1630-1680 with medium tomoiety strongis intensity. activity derivative in compound 7, 1 The complexity H to NMR spectra synthesized derivatives were in evaluated thisrelative derivative can be of used assess the role of The hydroxysemicarbazone moiety the tested of the synthesized compounds reflects the for their antibacterial activity against 6 different factderivatives. that due to the rigidity of C=N bond, the strains and the MIC values are listed in table 1. two substituents on the ketimine carbon give In addition to hydroxysemicarbazone derivatives As it appears in table 1, compound 7 showed the highest activity against P. aeruginosa rise to two different sets of hydrogens. This is 1-6, we were interested in bioactivity evaluation 1 observable in thewith H NMR compound 7. the Thissubstituted compoundbenzophenone has been and E. Coli IC50 spectra values of ofcompounds 62.5 and 31.25ofµg/mL. Among 1 and 4, where the aryl substituents on the reported to exhibit antitumor activity against derivatives, 2 with 2-carboxy was the active derivative especially ketimine carboncompound are the same (two phenyl ringssubstituent some cancer cell most lines (15). Since many antitumor in against compound 1 and two p-hydroxyphenyl have and alsoK. been reported to possess gram-negative strains such asrings E. Coli,derivatives P. aeruginosa pneumonia. While the 4in compound 4). In contrast to benzophenone antimicrobial activity (16-19), antimicrobial derivativein6 showed against Coli, P. aeruginosa, andmethoxyacetophenone 4,4’-dihydroxybenzophenone which moderate activity activity assessment of E. compound 7 as a the hydrogens on one ring are chemical shift previously established anticancer agent in order the acetophenone derivative 5 did not show any activity at 1000 µg/mL concentration. equivalent to their corresponding hydrogens to evaluate its potential to be considered as a lead on theBased other on ring, the 1H of NMR antimicrobial drugthe discovery seems logical. theinanalysis the spectra activityofdata, in it is suggested that presence of hydrophilic compounds 1 and 4 the chemical shifts for the Furthermore, since the hydroxysemicarbazone substituents on the ring is important for antibacterial activity. evident from the hydrogens of each ring aromatic is different from those moiety is replaced by a Asphenylhydrazine onactivity the otherdata, ring. As a result, two sets of double derivative in compound 7, thishigher derivative can bethan 2-carboxy substituted benzophenone derivative 2 showed activity doublets are observed in the aromatic region of used to assess the role of hydroxysemicarbazone 1 the unsubstituted benzophenone derivative 1. Similarly, 4-methoxysubstituted acetophenone H NMR spectra of compound 4. E/Z isomerism moiety in the tested derivatives. as another outcome of C=N bond rigidity As it appears in table 1, compound 7 showed accounts for the complexity of the 1H NMR the highest activity against P. aeruginosa and 33
Hariri E et al. / IJPR (2016), 15 (Special issue): 29-35
Table 1. Antibacterial activity of the synthesized derivatives. MIC (µg/mL)
Compounds 1
> 1000
500
250
250
1000
2
500
125
125
62.5
500
3
125
> 1000
500
> 1000
250
4
125
1000
500
1000
62.5
5
1000
1000
1000
1000
1000
6
500
500
250
125
500
7
500
125
62.5
31.25
500
4
0.5
2
1
2
≤4
≤4
32
≤4
≤2
+
+
+
+
+
Amikacin Vancomycin DMSO
Conclusion
E. Coli with IC50 values of 62.5 and 31.25 µg/ mL. Among the substituted benzophenone derivatives, compound 2 with 2-carboxy substituent was the most active derivative especially against gram-negative strains such as E. Coli, P. aeruginosa and K. pneumonia. While the 4-methoxyacetophenone derivative 6 showed moderate activity against E. Coli, P. aeruginosa, the acetophenone derivative 5 did not show any activity at 1000 µg/mL concentration. Based on the analysis of the activity data, it is suggested that the presence of hydrophilic substituents on the aromatic ring is important for antibacterial activity. As evident from the activity data, 2-carboxy substituted benzophenone derivative 2 showed higher activity than the unsubstituted benzophenone derivative 1. Similarly, 4-methoxysubstituted acetophenone derivative 6 exhibited higher potency than its unsubstituted analog (compound 5). It is notable that compounds 2 and 5 demonstrated some selectivity through inhibition of gram-negative bacteria. This might be due to higher penetration of these hydrophilic derivatives inside the gramnegative bacteria through their hydrophilic porin channels which leads to availability of higher toxic concentrations of the tested derivatives inside the microorganism. Besides the hydroxysemicarbazones 1-6, phenylhydrazone derivatives 7 exhibited promising antibacterial activity which gives us a new opportunity to conduct more focused studies on the antibacterial activity of other phenylhydrazone derivatives of diaryl systems.
In this study, based on the pharmacophoric elements of active hydroxythiosemicarbazones and semicarbazones, a group of hydroxysemicarbazone derivatives were synthesized. The synthetic yields of the diaryl ketone derivatives improved by employing either microwave technique, increasing the reaction time or addition of molecular sieve. The derivatives were analyzed by different spectroscopic methods and among them, the 1H NMR data confirmed the formation of some derivatives as a mixture of E/Z isomers. The antibacterial activity of the synthesized derivatives was assessed against 6 pathogenic bacterial strains and compounds 2, 6 and 7 exhibited the highest activity. Notably, the activity of these derivatives was more pronounced against gram-negative bacteria. The facts that both compounds 2 and 7 possess hydrophilic substituents and selectivity over gram-negative bacteria, suggest their possible higher capability to pass through the porin channels available on the cell wall of the microorganism as a reasonable explanation for their antibacterial activity. References (1) Kalinowski DS and Richardson DR. Future of toxicologys–Iron chelators and differing modes of action and toxicity: The changing face of iron chelation therapy. Chem. Res. Toxicol. (2007) 20: 715-720. (2) Dover LG, Alahari A, Gratraud P, Gomes JM and Bhowruth VE. EthA, a common activator of
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Antibacterial N-hydroxy semicarbazones
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